This invention relates to a weight sensor and, more particlarly, to a weight sensor capable of effectively eliminating low-frequency noise.
A weight sensor finds use in, e.g., a combinatorial weighing apparatus.
A combinatorial weighing apparatus operates by supplying a plurality of weighing machines with articles to be weighed, computing combinations based on weight values obtained from the weighing machines, selecting a combination giving a total combined weight value equal or closest to a target weight, and discharging the articles solely from those weighing machines corresponding to the selected combination, thereby providing a batch of weighed articles of a weight equal or closest to the target weight.
Such a combinatorial weighing apparatus will now be described in brief with reference to FIG. 1. The apparatus includes weight sensors M.sub.1, M.sub.2 . . . M.sub.n for sensing the weight of articles introduced into the respective weighing hoppers belonging to n weighing machines. Each weight sensor produces a weight signal, namely an analog value indicative of the weight sensed thereby. The weight signals from these weight sensors M.sub.1, M.sub.2 . . . M.sub.n are applied as multiple input signals S.sub.1, S.sub.2 . . . S.sub.n to a multiplexer 11. The multiplexer 11, which is composed of analog switches or the like, responds to a control signal S.sub.c from a computation controller 20, described below, by selectively applying the weight signals S.sub.1, S.sub.2 . . . S.sub.n as a weight data signal S.sub.o to a buffer circuit 12 sequentially in a time series. The buffer circuit 12 delivers the weight data signal S.sub.o received from the multiplexer 11 to a sample/hold circuit 13 upon subjecting the signal to an impedance conversion. The sample/hold circuit 13 repeatedly samples and holds the weight data signal S.sub.o subjected to the impedance conversion by the buffer circuit 12 and delivers the weight data signal to a buffer circuit 14. The latter subjects the signal to an impedance conversion, producing an analog weight data signal S.sub.p which is delivered to an analog-digital converter (A/D converter) 15. The latter digitizes the analog weight data signal S.sub.p to produce a digital output S.sub.d which is applied to the aforementioned computation controller 20. The latter is composed of a microcomputer and includes a processor 21 for performing combinatorial processing, a read-only memory (ROM) 22 storing a control program for combinatorial processing, and a random-access memory (RAM) 23 for storing the weight data as well as the results of processing performed by the processor 21. The computation controller 20 computes combinations on the basis of the weight data, selects a combination giving a total combined weight value equal or closest to a target weight, and delivers a drive signal to drive units H.sub.1, H.sub.2 . . . H.sub.n of respective weighing hoppers belonging to those weighing machines which correspond to the selected combination.
In a weighing apparatus relying upon electronic circuitry of the above-described kind, each of the weight sensors M.sub.1 -M.sub.n makes use of a scale cell comprising strain gauges which exhibit a change in resistance resulting from strain caused by the load of the articles being weighed, and a load-sensitive element on which the strain gauges are bonded. Since a weight sensor of such construction exhibits excellent detection sensitivity, there are situations where the weight sensor produces an erroneous weight signal which may be brought about by floor vibration caused by environmental conditions at the site of installation.
A technique for cancelling out a weighing error caused by such floor vibration is disclosed in the specification of Japanese Patent Publication No. 53-5823. As shown in FIG. 2, this example of the prior-art sensor includes a dummy cell DC provided in the vicinity of a scale cell SC to produce a noise signal ascribable to floor vibration, and an adder ADD the output of which is the output level of the scale cell SC. On the basis of the output level, the noise level included in the output signal of the dummy cell DC is made to agree with the noise level of the floor vibration noise included in the output signal of the scale cell SC. Then, the noise signal from the dummy cell DC and the output signal of the scale cell SC, (the noise levels of which are now in agreement) are added in opposite-phase relation by the adder ADD, thereby eliminating the floor vibration noise contained in the output signal of the scale cell SC. Such feedback control with respect to the noise level of the dummy cell DC is carried out for a specific reason. Namely, since the noise level of the floor vibration noise included in the output signal of the scale cell SC, varies in dependence upon the load of the articles acting upon the weight sensor, the noise level of the dummy cell DC must be made to change as a function of the noise level of the scale cell SC, for otherwise the noise levels of the two cells could not be made to agree at all times. This is the reason for feedback control. When such control is carried out, however, there is a pronounced decline in step response when an article to be weighed is applied to the weight sensor. Accordingly, feedback control cannot be applied to a high-speed weighing operation, as is performed by a combinatorial weighing apparatus. Another problem is that there is a decline in the reliability of the weighing apparatus in cases where a steady deviation or offset develops in the feedback circuitry, the reason being that such a deviation manifests itself as a weighing error.